1. Field of the Invention
The present invention relates to an image forming apparatus which includes an image forming unit and an image reading unit that is arranged on the image forming unit and has an image reading portion therein.
2. Description of the Related Art
An ordinary image forming apparatus is provided with an image reading unit having therein an image reading portion which reads out an image formed on an original, and an image forming unit having therein an image forming portion which forms an image on a sheet, and has the image reading unit arranged on the image forming unit. The image reading portion includes a printed circuit board (image sensor substrate) having a photoelectric conversion element which exposes the original to light while scanning the original and thereby captures image information therein.
The image forming unit has an image processing portion which is arranged on a rear side of a casing and performs image processing. This image processing portion includes a printed circuit board (image processing substrate) that is a printed wiring board which has semiconductor devices such as a CPU mounted thereon.
The image reading portion and the image processing portion are connected by a cable, and the image processing portion acquires an image signal from the image reading portion through the cable. An original pressing plate for bringing an original into pressure contact with a platen is arranged on the image reading unit, and the original pressing plate can be freely opened and closed by a hinge mechanism.
In recent years, this original pressing plate is often incorporated in an original feeder unit which automatically feeds the original to the image reading unit. The hinge mechanism needs to freely open and close the original feeder unit which is a heavy article while supporting the original feeder unit and is, therefore, formed out of an electrically conductive member made of metal or the like.
Here, since an image signal (digital signal) is transmitted to the cable which connects the image reading portion to the image processing portion, it is required to suppress a radiation noise from the cable that is generated when the image signal is transmitted.
To suppress the radiation noise, the end of the lower part on the rear side of the electrically conductive casing of the image reading unit is connected to the end, opposite to the former end, of the upper part on the rear side of the electrically conductive casing of the image forming unit by an electrically conductive connecting member, and the cable is arranged on the connecting member. This connecting member functions as a path of a return current in response to a signal current passing through the cable. The radiation noise from the cable is suppressed by arranging the cable in the proximity of the connecting member.
However, the hinge mechanism is formed out of the electrically conductive member as described above and is a movable portion. For these reasons, it is difficult to cover the hinge mechanism wholly with an outer sheath such as a resin member, resulting in the exposure of the electrically conductive member. This causes electrostatic discharge (Electrostatic Discharge: ESD) to easily invade the casing of the image reading unit from the hinge mechanism.
The noise current caused by the ESD invading the casing of the image reading unit passes through the connecting member, and propagates through the casing of the image forming unit. At this time, the noise current propagates through the cable by electromagnetic coupling with the cable arranged in the vicinity of the connecting member, and may cause degradation of the image signal transmitted by the cable. Furthermore, in recent years, the transmission rate of transmitting this image signal becomes extremely high because of the accelerated image reading and image forming rates of the image forming apparatus and the improved definition of the image. As a result, the image signal is more susceptible to the noise such as the ESD.
To cope with these problems, Japanese Patent Application Laid-Open No. 2011-146150 proposes a method of making a cable less susceptible to noise such as ESD by shielding the cable.
However, the shielded cable described in Japanese Patent Application Laid-Open No. 2011-146150 has a problem of high cost.
Moreover, if the shielded cable is applied to the cable which connects the image reading portion to the image processing portion in the image forming apparatus, new problems occur. Specifically, the image reading portion needs to mechanically scan the original when reading out the original, and it is, therefore, required for the cable through which the image signal is transmitted to have a slidably flexing performance. In recent years, the image reading portion is thinner and the movable area of the cable which moves along with the scanning of the image reading portion is also limited, accordingly. Owing to this, the cable is required to have a higher slidably flexing performance. However, the shielded cable is disadvantageously unable to provide a sufficiently high slidably flexing performance since the shielded cable includes a shielding layer.